Method and equipment for manufacturing predetermined low bark content wood chips and a high bark content fuel fraction from wood chips with bark attached

ABSTRACT

A method and equipment manufacturing predetermined low bark content wood chips and a high bark content fuel fraction from wood chips with bark attached, includes mechanical bark removal treatment, in which the bark is removed from the chips and their size is reduced; pre-cleaning of the flow of chips with bark attached, into a flow of chips with a bark content of less than 10% and high bark content rejects, with the aid of pneumatic and mechanical sieving; final cleaning of the aforesaid wood chip flow from the pre-cleaning, with the aid of an optical separator, to create low bark content wood chips and a fraction with a higher bark content; collection of the rejects leaving the process, to create the aforementioned high bark content fuel fraction; and seiving out of the flow of wood chips, before the mechanical bark removal process, a predetermined thin fraction of the chips, which bypasses the debarking process.

FIELD OF THE INVENTION

This invention relates to a method and equipment for manufacturingpredetermined low bark content wood chips and a high bark content fuelfraction from wood chips with bark attached. More specifically themethod includes a mechanical bark removal process, pre-cleaning of woodchips with bark attached, and final cleaning of the flow of wood chips,with the aid of an optical separator.

BACKGROUND OF THE INVENTION

A wood bark manufacturing method is known from international patentpublication WO 93/25324. According to this, the wood chips with barkattached are ground initially either with a special plate grinder, orwith a vibrating cone crusher, wherein the bark separates from the woodchips and the size of the bark particles diminishes. Thereby, the barkcan be separated more easily during later separation stages. A grinderor a vibrating cone crusher is followed by pneumatic separation, toseparate outer birch bark, after which sawdust is removed using avibrating sieve. The bark content has now dropped to considerably lessthan 10%, and the final cleaning can be carried out using an opticalsorter. According to the patent, two optical sorters are used in series,but can, however, be replaced by a single more powerful device. Theaccepted fraction from the second optical sorter is led to grinding andthe reject to the fuel fraction.

Equipment that is essentially that described by the application has beenbuilt in Kankaanp{umlaut over (aa)} in Finland, the supplier being BMHWood Technology Oy. In the commercial equipment, a magnetic separatorand a pneumatic separator are used in the pre-cleaning to separate metalscrap and stones. Sawdust is removed from the commercial equipmentbefore grinding, and after grinding the vibrating sieve is replaced by aso-called pocket-roll sieve (a roller sieve developed especially forsieving sawdust). In the commercial equipment, a good yield is achievedwith a bark content of about 1%, which is sufficiently clean for themanufacture of certain grades of cellulose. The yield varies between60-70%, depending on the species of timber and other factors. If cleanerchips are wanted, the yield drops, and correspondingly improves with apoorer level of cleanliness.

SUMMARY OF THE INVENTION

An object of the present invention is to achieve a better yield thanpreviously at each corresponding level of cleanliness.

The characteristic features of the method for manufacturingpredetermined low bark content wood chips and a high bark content fuelfraction from wood chips with bark attached, includes the steps of:

mechanical bark removal treatment, in which the bark is removed from thechips and their size is reduced,

pre-cleaning of the flow of chips with bark attached, into a flow ofchips with a bark content of less than 10% and high bark contentrejects, with the aid of pneumatic and mechanical sieving,

final cleaning of the aforesaid wood chip flow from the pre-cleaning,with the aid of an optical separator, to create low bark content woodchips and a fraction with a higher bark content, and

collection of the rejects leaving the process, to create theaforementioned high bark content fuel fraction, characterized in thatbefore the mechanical bark removal process, a predetermined thinfraction of the chips, is sieved out of the flow of wood chips, and thenbypasses the debarking process.

The mechanical bark removal process may take place by means of agrinder, press-rollers, or a crusher. Alternatively, the mechanical barkremoval takes place using a blade ring chipper, which cuts the woodchips into smaller pieces, while removing the bark.

According to the method, 20-100% of the fraction with a higher barkcontent obtained from the optical separator is directed to the fuelfraction, and correspondingly 80-0% is returned to the mechanical barkremoval process. The optical separator includes a pneumatic conveyor anda division plate, by means of which the flow of chips is divided firstinto the said wood chips with a low bark content and wood chips with ahigher bark content and further the latter part is divided with the aidof the division plate directly into a fuel fraction and the said partthat is returned.

The method may include a pre-separation step before the thicknesssieving step, in which the oversized wood chips, stones, and metals areremoved from the wood chips with bark attached.

The flow of wood chips from the mechanical bark removal process and thethin chip fraction from the thickness sieving may be sieved using amechanical sieve, to remove sawdust as a third reject and using apneumatic separator to remove light birch bark, leaves, needles, andother airborne material as a fourth reject.

Equipment for manufacturing wood chips with a low bark content from woodchips with bark attached, which equipment includes mechanical barkremoval devices and pre-separation devices consisting of a mechanicaland a pneumatic separator and an optical sorter. The equipment ischaracterized in that it includes a wood chip thickness sieve locatedbefore the mechanical bark removal devices and a correction lineconnected to the exit of the thin fraction obtained, by means of whichthe thin fraction is taken past the mechanical bark removal devices tothe pre-cleaning devices.

The mechanical bark removal devices may consist of at least one of ablade ring chippers, a grinder, a press roller, and a crusher.

The distribution values of the wood chip fractions according to theaccompanying table are, to a great extent, the point of departure of theinvention:

Sieve Fraction Clean Wood Bark % of gap remaining wood chips chips woodchips mm in sieve % in fraction with bark with bark 0  5,03 2 16,7379,03  1,96  3,61 4 37,94 89,98  2,72  8,57 6 25,37 85,54  7,15 16,78 8 8,01 74,54 14,83 19,98 10   6,92 69,00 25,51 18,30

It is surprising, that in the smallest fractions (sieve gap 4 mm, orless), there is not much bark attached to the chips, whereas in chipsabove this size the proportion of chips with bark attached isconsiderable. Because mechanical processing in any event reduces thesize of the wood chips across the board, it is advantage s to separatethis fairly clean part of the wood chips and have it bypass themechanical processing. There is bark as such even in the smallest chips,but it is loose, and most of it leaves during the separation of thesawdust, which has been moved so that it takes place after themechanical processing.

These and other features and advantages of the invention will be morefully understood from the following detailed description of theinvention taken together with the accompanying drawings, which showschematically one plant according to the invention and the method usedin it.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings:

FIG. 1 illustrates the method according to the invention, as a blockdiagram; and

FIG. 2 illustrates equipment according to the invention, as a plantdiagram.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to the drawings in detail, in the diagram in FIG. 1, thewood chips with bark attached can be considered as having been obtainedby conventional methods, using a known cutter. In the first stage ofprocessing, the wood chips are cleaned in pre-separation 3, 4, 5, whichcomprises, among other things, magnetic devices for removing scrap ironand a pneumatic separator 4, 4′ for removing stones and sand. Inpre-separation, oversize wood chips are separated by means of disc sieve5, and in practice are most advantageously led to the fuel fraction, butif necessary they can be led, for example, to a crusher and then back topre-separation. The fraction accepted in pre-separation 3, 4, 5 is ledto thickness sieving 6, which is intended to separate the thin wood chipfraction, generally less than 6 mm, from the part of the flow of woodchips travelling through the thickness sieve. This part is led to thefollowing stage, i.e. past mechanical bark separation 7. Mechanical barkseparation can take place by means of either a plate grinder, a crusher,or in a blade ring chipper, which has been shown to be the mostadvantageous, because it does not defibrate the wood materialunnecessarily.

The mechanically processed fraction, together with the smaller fractionobtained from the bypass line, is led to pocket-roll sieve 9, whicheffectively removes the sawdust from the flow of material. After this,the accepted fraction is led to a pneumatic separator, to remove lightbirch bark, leaves, needles, and other easily airborne particles fromthe flow of material. After this, the pre-cleaned wood chips alreadyhave a bark content of less than 10%, and can be led to optical sorter10, from which wood chips with a bark content of about 1% can beobtained, even at a large yield. Reject from the optical sorter is ledthrough selection 15 in a regulated manner either to the fuel fraction,or back to mechanical processing. In order to prevent a so-called ‘madcycle’, 20-100% is always led to the fuel fraction, and only 80-0% isled to the mechanical processing. Certain species of timber or otherconditions will mean that there is not necessarily any benefit frommechanical processing, in which case it is better to lead the rejectfrom the optical sorter directly to the fuel fraction.

The plant diagram of FIG. 2, shows cutter 1 and reception funnel 2, inwhich the wood chips are collected, or into which wood chips fromexternal sources are poured. From here the flow of wood chips is ledthrough magnetic separator 3 to disc sieve 5. Between these there is apneumatic separator 4′, which removes stones and a second pneumaticseparator 4, which sucks light birch bark, leaves, and fine dust intothe airflow, see also stone store 3.1. A disc sieve 5 removes oversizedpieces from the wood chips, which are led to the fuel fraction 13,together with reject from the pneumatic separator. Beneath the discsieve there is a thickness sieve 6, which separates the smallestfraction and the sawdust in it, these being led to mechanical barkseparation, going past grinder 7, whereas the rest of this part is ledto grinder 7. Experiments have, however, shown that bark can beeffectively separated by cutting the chips. The mechanically processedchip fraction obtained from the plate grinder and the thin fractionobtained from the bypass line 8 are led to the Pocket-Roll sieve 9,which removes sawdust and fine bark material from the flow of chips.After the Pocket-Roll sieve, there is a pneumatic separator 11, whichremoves light birch bark, leaves, needles, and other easily airbornematerial.

The accepted fraction is led to optical sorter 10, from which a goodyield of good quality cellulose wood chips 12 are obtained. The rejectis led through selection member 15, either to the fuel fraction 13 or tothe grinder 7. Using selection member 15, 0-100% can be taken from theflow of materials into the fuel fraction. It is advantageous to take atleast 20%, to prevent a so-called ‘mad cycle’ in the process. Withcertain grades of wood chips part of the material would remain tocirculate continuously through the process, unless part of the reject isremoved to the fuel fraction. All of the removed rejects are led in thefigure to fuel fraction 13, by means of conveyor 16.

Because the optical sorter uses a pneumatic conveyor, the reject flow isnot homogeneous. A divider plate 18 set in the exit area can separatethe part with the greatest bark content from the rest of the reject, itthen being advantageously led directly to the fuel fraction with the aidof conveyor 19. The figure shows this alternative by broken lines.

The pulverization of the wood is reduced with the aid of the methodaccording to the invention, which increases the cellulose chip yieldfrom the previous yield by 5-10 percentage units, while the fraction ofcellulose chips remaining in the 13 mm particle size perforated sieveincreases substantially (by 15-35 percentage units).

In equipment according to the invention, a thickness sieve is used toseparate the fraction with the desired value, 4-8 mm, for example,thinner than 6 mm, which is led past the barking separation and thethicker wood chips are fed to the mechanical bark separation. Thesefractions are combined and the sawdust and light birch bark is sievedout and sorted optically, the accepted fraction of which is clean woodchips and the reject is returned to mechanical bark separation, in whichthe bark is ground to a smaller size, so that it can be distinguished inthe sawdust discharge.

Improvement of the cellulose wood chip yield by 5 percentage unitsimproves the profitability of the plant by about FIM 20/m³, whichrepresents FIM 3 000 000 p.a. in a plant producing 150 000 m³ p.a.. Theeffect of the chip size of cellulose wood chips on the sales pricevaries from case to case, but at its greatest is about FIM 25/m³.Together these correspond, in magnitude, to the operating and capitalcosts of the plant, so that the economy of the method improvessubstantially.

Although the invention has been described by reference to a specificembodiment, it should be understood that numerous changes may be madewithin the spirit and scope of the inventive concepts described.Accordingly, it is intended that the invention not be limited to thedescribed embodiment, but that it have the full scope defined by thelanguage of the following claims.

What is claimed is:
 1. A method for manufacturing predetermined low barkcontent wood chips and a high bark content fuel fraction from wood chipswith bark attached, comprising the steps of: sieving the wood chips withbark attached into a predetermined thin wood chip fraction and sievingoverflow chips; mechanically removing bark from the sieving overflowchips, thereby producing smaller overflow chips, and pre-cleaning, withthe aid of pneumatic and mechanical sieving, the predetermined thin woodchip fraction and the smaller overflow chips, into a flow of chips witha bark content of less than 10% and a first fraction of higher barkcontent rejects; final cleaning of said flow of chips with a barkcontent of less than 10%, with the aid of an optical sorter, to createlow bark content wood chips and a second fraction of higher bark contentrejects; collecting at least part of said first and second higher barkcontent rejects to create said high bark content fuel fraction.
 2. Themethod according to claim 1, characterized in that the step ofmechanically removing bark from a flow of chips having bark attachedtakes place by means of a grinder, press-rollers, or a crusher.
 3. Themethod according to claim 1, characterized in that the step ofmechanically removing bark from a flow of chips having bark attachedtakes place using a a blade ring chipper, which cuts the wood chips intosmaller pieces, while removing the bark.
 4. The method according toclaim 1, characterized in that 20-100% of the second fraction of higherbark content rejects obtained from the optical sensor is directed to thefuel fraction, while 80-0% is returned to the step of mechanicallyremoving bark.
 5. The method according to claim 4, characterized in thatthe optical sensor includes a pneumatic conveyor and a division plate,by means of which the flow of chips with a bark content of less than 10%is divided first into said low bark content wood chips and said secondfraction of higher bark content rejects, said second fraction of higherbark content rejects being further divided with the aid of the divisionplate directly into a fuel fraction, with a remainder of said secondfraction of higher bark content rejects being returned to the step ofmechanically removing the bark.
 6. The method according to claim 1,characterized in that it includes a pre-separation step before thesieving step, in which oversized wood chips, stones, and metals areremoved from the wood chips with bark attached.
 7. The method accordingto claim 1, characterized in that the predetermined thin wood chipfraction and smaller overflow chips are sieved using a mechanical sieve,to remove sawdust as a third reject and by using a pneumatic separatorto remove light birch bark, leaves, needles, and other airborne materialas a fourth reject.
 8. Apparatus for manufacturing wood chips with a lowbark content from wood chips with bark attached, said apparatuscomprising: a wood chip thickness sieve for separating the flow of woodchips with bark attached into an outflow of a predetermined thin woodchip fraction and an outflow of sieving overflow chips; a mechanicalbark removal device functionally integrated with said wood chipthickness sieve; pre-cleaning devices functionally integrated with saidmechanical bark removal device, said pre-cleaning devices comprising amechanical and a pneumatic separator and an optical sorter; saidapparatus further including a correction line functionally integratedbetween said wood chip thickness sieve and said pre-cleaning devices toreceive said outflow of said predetermined thin wood chip fraction sothat said predetermined thin fraction bypasses said mechanical barkremoval devices and flows directly to the pre-cleaning devices. 9.Equipment according to claim 8, characterized in that the mechanicalbark removal devices consist of at least one of a blade ring chipper, agrinder, a press roller, and a crusher.